#app4{

}

@keyframes change{
    0%{
        background: red;
    }
    100% {
        background: blue;
    }
}

#app4 .circle{
    width: 30vw;
    height: 30vh;
    border-radius: 50%;
    border: 1px solid red;
}

#app4 .circle.active{
    animation: change 1s infinite alternate linear;
}